It is well known in the art of visual projectors systems to use a spatial light modulator ("SLM") positioned in a light stream. The SLM is a semi-transparent device that contains a pattern of clear and opaque regions that modify the light stream to form a projected image. In particular, the SLM consists of numerous small areas (pixels) of controllable light transparency that are electronically adjusted to produce the projected image from the light stream.
In one type of SLM, a liquid crystal modifies the light emissions from the projection system at each pixel. Transmission of light through a liquid crystal depends on the polarization state of the liquid crystal, which may be adjusted to either transmit or block light to form the equivalent of a bright or dark spot in the output. The polarization state of the liquid crystal can be electronically controlled to allow very accurate control of the light emissions. Because the liquid crystals defining the pixels are relatively small and because the electronic control allows precise control of the liquid crystals, the resulting projected image may be very accurate and sharp.
Alternatively, each pixel employs a digital mirror to modify the light emissions. The digital mirror consists of a movable mirror in which the light is either reflected toward or away from the screen, thus forming the bright or dark spots. Again, the positioning of the digital mirrors is electronically controlled to allow very accurate control of the light emissions.
Accordingly, the use of the SLM in an image projection system is advantageous because it allows precise electronic control of light emissions through the pixels. Thus, a projection system containing an SLM may produce a precise, high-resolution projected image.
However, the performance of an SLM projection system depends critically on the collection and focusing of light energy from the lamp to the SLM. In particular, to illuminate the spatial modulator and project the output onto the screen, it is necessary for the light to be uniform over the SLM and to have sufficient amount brightness.
There are several known systems for collecting and condensing light from a light source, such as a lamp, in a projection system. In an "on-axis" system, the light source and the target are located on the optical axis. In these on-axis systems, it is known to use one or more reflectors having either an ellipsoidal and parabolic shape, together with an imaging lens to direct the light from the light source. However, the on-axis systems suffer from the basic limitation of losing brightness when coupling the light source to the SLM. This loss of brightness degrades the overall efficiency and performance of the projection system.
U.S. Pat. No. 4,757,431 ("the '431 patent") describes an improved light condensing and collecting system employing an off-axis spherical concave reflector to enhance the flux illuminating a small target and the amount of collectable flux density reaching the small target. A further improved light condensing and collecting system is provided by U.S. Pat. No. 5,414,600 ("the '600 patent), which discloses the use of an ellipsoid concave reflector. Similarly, U.S. Pat. No. 5,430,634 ("the '634 patent) discloses the use of a toroid concave reflector. The systems of the '431, the '600 and the '634 patents provide a near 1 to 1 (magnification free) image and conserve brightness from the light source. However, these systems lose the 1 to 1 (unitary) magnification, which degrades overall projection system performance, as the amount of collected light is increased by raising the collection angle of the reflector. Therefore, in these systems, increasing the brightness of illumination decreases the quality of the produced image.
In the related field of spectroscopy, there is also a need to collect and condense light from a light source. In particular, light from a light source is focused into at a sample. The sample is then tested by collecting and evaluating the radiation from the sample.
In spectroscopy, it is common to use parabolic-shaped reflecting mirrors in off-axis reflecting systems to focus the light emissions from the light source. For instance, U.S. Pat. No. 3,986,767 describes a parallel beam being focused into a small spot directly onto the test sample using an off-axis paraboloid reflector. Similarly, U.S. Pat. No. 4,591,266 (Re 32,912) discloses a spectroscopy system that uses a matched pair of off-axis paraboloid reflectors that have their foci optically imaged on the sample, either at a common point or at two points which are optically imaged on each other, and having relative locations and orientations such that each ray of radiation from the light source strikes the two reflectors at points on the reflectors having approximately the same focal lengths. U.S. Pat. No. 4,473,295 illustrates the configuration of another spectroscopy system using paraboloids to collect and focus light onto a test sample.
Similarly, U.S. Pat. No. 5,191,393 ("the '393 patent") and corresponding European Patent No. EP 0 401 351 B1 relate to the transmission of light from the outside of a cleanroom into the cleanroom for optical measurement of small features. One of the configurations presented in the '393 patent is the use of an arc lamp, two paraboloids reflectors, a single fiber, and the use of transmissive dichroic filters for filtering the needed wavelengths.
The use of off-axis paraboloid, as described in the above-cited patents, intrinsically does not provide efficiently coupling from the light source to the output target.
Therefore, there remains a need for a methodology of coupling light from a small source to a projection system that overcomes these disadvantages.